People will still use 270 STR accessoires after 4.05 Good Job square enix.

[Shurrikhan]

But in all honesty, people fail to realize that even with a full group comp, you lose out on the plethora of utilities that the DPS possess. A simple solution off the top of my head is have the DPS possess the damage buffs so that a proper tank and dps composition would have a much, much higher rdps than a full tank comp. This is already present in Limit Break. Balance content around the proper composition rdps and you will naturally have people playing all roles. Tanks can now have equal impact on fights and feel comparatively powerful? We can all compete against eachother in DPS? I'd extent this concept to healers as well but they can't heal and dps at the same time and have to actually choose their GCDs.
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Tanks being unpopular due to rewarding difficulty is much better than it being an unrewarding chore that only exists because content or design forces it.

I'd still rather be able to contribute through something other than JUST dps at that point, as a tank. I'd like to have to choose whether I want to help go ham on something or I want to contribute in some other way at a given time, not through long-term stretches such as GCD stance swaps and gauge-halving, or even the mere fact that I am a specialist (tank), but through more GCD-to-GCD or resource-consuming oGCD options.

The main issue, imo, is simply that so much of tanks' arsenals is content-scaling, meaning that their contribution inevitably makes them required at first and inferior later. They can be undertuned or overtuned to make that required phase longer, or vice versa, but as long as their mitigation and tools are consistently percentiles of enemy stats, rather than their own, they will not scale in the same way that DPS or healers will. The only notable exceptions to this was 1.x - 2.0 Warrior, with mere traces remaining in its current iteration. It was capable of no-healing Amdapor Keep myth runs, but painfully inferior to Paladin in first Coil, all because the one's toolkit scaled only with content, and the other's only with gear.

Build new mechanics for core tanking mechanisms that allow perhaps for some greater value in practice when still underleveled for content but that nonetheless scale as well as healing, and voila, you've got consistent tank relevance.